A semiconductor device may include gate dielectric layers respectively having thicknesses different from each other by a replacement metal gate (RMG) process called a “gate last process.” A high-voltage region may need a relatively thick gate dielectric layer and a low-voltage region may need a relatively thin gate dielectric layer. The semiconductor device including the gate dielectric layers having different thicknesses may be manufactured in the high-voltage region and the low-voltage region by the gate last process.